Radio propagation is the behavior of radio waves when they are transmitted, or propagated, from one point to another point. Radio waves will during their propagation be affected by several factors, such as e.g. reflection, diffraction, absorption and scattering, depending on the environment between the two points. Furthermore when transmitting from a network node 110, to a UE 130, the propagation may be along a direct line of sight path as illustrated in FIG. 1a, where a beam 120 is transmitted from a network node 110 to a UE 130 at an elevation angle δ. The propagation may also be along non-direct line of sight paths created for instance by reflections from different scatters. Non line of sight paths are crucial in any communication system that does not purely rely on line of sight transmissions, which is typically the case in urban areas. Another kind of effect that creates non-line of sight propagation is diffraction, illustrated in FIG. 1b which is the result of radio waves being bent around sharp edges, such as the beam 125 transmitted at the elevation angle φ over the building 140.
In general, when transmitting from the network node 110 to the UE 130, the radio propagation is not only along a single distinct path between the network node 110 to the UE 130, but rather along a set of paths, corresponding to a multipath propagation environment. These paths may correspond to different delays, spatial directions, and constitute different receive powers etc. More generally speaking, there is typically a continuum of paths that is dominated by certain primary directions.
Beamforming is a technique for directional signal transmission and/or reception. This is achieved by controlling the phase and amplitude of different signals transmitted from and/or received at spatially separated antenna elements e.g., oriented as a linear array, or vertically as the sub-elements of an active antenna. See for example the active antenna 210 in FIG. 2 and the example antenna subelements 230 and 240 producing signals having different polarizations.
In the case that the active antenna is mounted in such a way that antenna subelements are spread out on a vertical axis, one possible beamforming technique is “elevation beamforming” meaning that the transmitted and/or received signal is directed in the elevation domain. See the active antenna 210 in FIG. 2 with the active elements x(1) . . . x(NS) vertically arranged along the vertical antenna axis 220. Beamforming in this configuration is achieved using different phases and amplitudes for the different subelements of the active antenna such that at certain angles, relative the active antenna, the different signals experience constructive interference whereas at other angles they experience destructive interference.
One example of beamforming in the case of transmission is illustrated in FIG. 3a. As can be seen the network node 300 is able to dynamically direct its transmitted energy into three different directions; that is, beams, A-C. Furthermore, when the network node 300 is transmitting to the illustrated UE 310 the choice of using beam B has the advantage that the transmitted energy will be directed in the same direction as the direct propagation path between the network node 300 and the UE 310. This has the effect that the UE 310 receives a stronger signal from the network node 300. In the case that there were no dynamic elevation beamforming the network node 300 would instead need to use the same beam for all transmission and therefore not be able to dynamically focus the transmitted power in the direction towards its UE 310 of interest.
In FIG. 3b it is illustrated that the suitable direction for transmission is not necessarily the same as the direction that would correspond to the line of sight direction between the network node 300 and the UE 310. Here, transmission using beam A at an elevation angle α maximizes the received power at the UE 310, since it coincides with the direction of the propagation path from the network node 300 to the UE 310. If beams B or C with the elevation angels β or γ were used for transmission, the result would likely be a lower received power at the UE 310.
It should be emphasized that in a more general setting there may be any number of beams. In fact, the used beams may even be created dynamically pointing in an arbitrary elevation direction and with an arbitrary shape (e.g., width) implying an infinite number of possible beams.
The signal quality achieved with a given beam may be acquired or estimated by the network node in many ways, including power measurements in the uplink based on sounding signals from a UE or from feedback from a UE that, for example, measures received power of a set of reference signals transmitted by the network node. This beamforming technique applied in the elevation domain may be called dynamic elevation beamforming. The network node may also determine the signal quality for the beam candidates from data or control transmissions made by the UE in the uplink.
Although dynamic elevation beamforming is a powerful tool for directing the transmitted energy towards the UE of interest and may potentially increase the signal to noise ratio at one or more UEs in one cell, it may do so at the expense of lowering the signal to noise ratio in other UEs in neighboring cells due to interference created by the beams.